Such volumetric devices are used both when accurately dispensing liquid, especially corrosive media, and when titrating. They are designed to deliver quantities of liquids in the range of microliters to liters.
German patent 26 47 206 C3 discloses a dosing system for liquids essentially consisting of a piston valve pump and a valve block and affixable to a reservoir. The valve block comprises a discharge valve and an intake valve. Both valves are gravity-actuated ball-valves parallel to each other and mounted essentially vertically in the valve block. The intake valve includes a suction line going to the bottom of the reservoir. The discharge valve passes through a discharge stub to the outside. To withdrawn a specific quantity of liquid, the intake valve is opened by the suction stroke of the piston pump, the discharge valve being closed at the same time, and thereby the dosing cylinder will be filled. If the intake valve is then closed by a reversal in the motion of the piston valve pump, then the dosing cylinder shall be drained through the opened discharge valve and through the discharge stub into a collecting vessel.
Such a dosing instrument incurs the drawback that as a rule a partial vacuum is created in the reservoir upon removal of liquid from it, so that further withdrawal is hampered and may ultimately become impossible. For that reason the valve block screwed in a liquid- and gas-tight manner to the reservoir is provided in practice with a narrow ventilation duct connecting the inside of the reservoir directly to atmosphere. This design too was found to have practical shortcomings. After pressure balance between the reservoir and atmosphere has been reestablished upon withdrawal of liquid, for example, particles of liquid in this reservoir may enter the atmosphere (e.g., by diffusion etc.), in an uncontrolled and nearly unhampered manner with such dosing devices being preferentially used for exceedingly chemically corrosive media, for instance hydrochloric acid and nitric acid, this behavior is ecologically undesirable. Moreover, and especially in clean-room laboratories, uncontrolled discharge of even tiny amounts of gas leads to corrosion of high-sensitivity instrumentation and measurable contamination of many laboratory substances.
In view of this state of the art, it is the object of the invention to create a volumetric device with a reciprocating piston to deliver defined quantities of liquids, which comprises a simple ventilation means for pressure balance between the atmosphere and the reservoir but which simultaneously also assures reliable protection against undesired emission of even the least amount of liquid or gas particles from the reservoir.